1. Field of the Invention
This invention concerns diodes and their use and fabrication. More particularly, it concerns diodes which include semiconducting conjugated polymers. These diodes have both light-emission and photo-detection properties and may be used in displays and the like.
2. Brief Description of the Prior Art
Solid-state light-emitting diodes ("LEDs") have found widespread application. Conventional LEDs are fabricated for inorganic semiconductors such as gallium arsenide typically doped with aluminum, indium or phosphorus.
Since about 1989, interest has been drawn to LEDs fabricated from semiconducting organic polymers. In 1989, coinventor A. J. Heeger and others described diodes formed from polymers (H. Tomozawa, D. Braun, S. D. Phillips, R. Worland, A. J. Heeger and H. Kroemer, Synth. Met. 28, C687 (1989)) and in a latter paper (D. Braun and A. J. Heeger Appl. Phys. Lett. 58, 1982 (1991) pointed out that these diodes emitted light. In 1990, others (H. Burroughs, D. D. C. Bradley, A. R. Brown, R. N. Marks, K. Mackay, R. H. Friend, P. L. Burns and A. B. Holms, Nature 347, 539) described a light-emitting diode based on conducting organic polymers.
These conductive polymer-based materials offer promise because they present superior fabrication possibilities coupled with reasonable conversion efficiency. Using indium/tin oxide ("ITO") and the semiconducting polymer, poly(2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylene vinylene), ("MEH-PPV") in a Ca/MEH-PPV/ITO layered thin film device, D. Braun and A. J. Heeger (Appl. Phys. Lett.) demonstrated light emission at an external energy conversion of 1% (photons/electrons).
By using a conducting polymer, rather than indium/tin-oxide, as the hole-injecting contact, robust and mechanically flexible LEDs have been demonstrated (G. Gustafsson, Y. Cao, G. M. Treacy, F. Klavetter, N. Colaneri, and A. J. Heeger, Nature 337, 477 (1992)). Recent work has shown that by generalizing to heterojunction devices which include an electron transport layer, device efficiency can be improved in polymer LEDs using BCHA-PPV (C. Zhang, S. Hoger, K. Pakbaz, F. Wudl and A. J. Heeger, J. Electron. Mater. 22, 745 (1993)) and in polymer LEDS using poly(phenylene vinylene), ("PPV"), with air-stable electrodes for example, aluminum and indium. (R. Friend, Oral presentation at the Minisymposium on Polymer Light Emitting Diodes, Eindhoven, Sep. 15-17, 1993)
It has recently been reported by several groups that diodes fabricated from poly(phenylene vinylene) give a photovoltaic response. (S. Karg, W. Riess, V. Dyakonov and M. Schwoerer, Synth. Met. 54, 427 (1993); H. Antoniadis, B. R. Hsieh, M. A. Abkowitz, S. A. Jenekhe and M. Stolka, Synth. Met. (in press)) This is consistent with experience with inorganic semiconductors which are typically "light sensitive to some degree." McGraw-Hill Encyclopedia of Science and Technology 13, 379 "Photodiode," 1987 .